BIODIVERSITY OF THE ZAMBEZI BASIN Jonathan Timberlake 2000 Occasional Publications in Biodiversity No. 9 Biodiversity Foundation for Africa P.O. Box FM730, Famona, Bulawayo, Zimbabwe Biodiversity of the Zambezi Basin 1 BIODIVERSITY OF THE ZAMBEZI BASIN Jonathan Timberlake 1. INTRODUCTION The Zambezi River Basin lies across southern tropical Africa, with significant portions in Angola, Zambia, Zimbabwe, Malawi and Mozambique and smaller portions in Namibia, Botswana and Tanzania (see Figure 1). Its total extent is approximately 1.33 million km2 (Hughes & Hughes 1992). The Zambezi River itself rises in northwestern Zambia on the Central African Plateau which covers much of the continent. It first drains south then eastwards, and exits into the Indian Ocean north of Beira in Mozambique. The catchment of the main river and a myriad of tributaries – such as the Kafue, Kwando, Gwayi, Manyame, Mazoe, Luangwa and Shire – is underlain by ancient rocks of Triassic and Jurassic age (more than 120 million years old). Although the majority of the basin is covered by woodland and savanna vegetation, interspersed with grasslands and wetlands where drainage is poor, from a biodiversity perspective there are small but significant areas of other habitats such as montane grasslands and shrublands, forests and deep water lakes. In this paper an outline of the evolution of the Zambezi Basin is given, followed by an account of our knowledge on the vegetation types, plants, vertebrates and invertebrate species found there. Particular reference is given to those species of ecological or economic significance. Areas and species of particular interest from a conservation viewpoint are mentioned, and the conservation threats are outlined. 2. THE ZAMBEZI BASIN 2.1 Zambezian Biomes In broad biodiversity terms there are four main biomes across the basin – a Congolian biome, a Zambezian biome, a Montane biome and a Coastal biome (Figure 1, Table 1). (a) The Congolian biome is that area associated with the headwaters of the Zambezi in northwestern Zambia and northeastern Angola with a moister and warmer climate than that of the rest of the plateau portion of the basin. Here the vegetation and species are a mix of those found in the forested Congo Basin and those found in the less tropical, more wooded, Zambezi Basin. Biodiversity of the Zambezi Basin 2 Biodiversity of the Zambezi Basin 3 (b) The Zambezian biome covers the great majority of the basin, around 95%, and comprises woodland, grassland, swamp and lakes. The climate is strongly seasonal with a marked dry season. Frost often occurs on flat ground on the plateau. This biome is sometimes subdivided into moister areas with miombo broad-leaved woodland, and drier areas with mopane or Acacia woodland. (c) The Montane biome lies above 1800-2000 m altitude and is cooler, wetter, often shrouded in mist, and with a much more temperate climate. The vegetation and species are generally similar to those found along the Eastern escarpment mountains, the series of mountains which stretch from Ethiopia through Kenya, Uganda and Tanzania down to the Drakensberg in South Africa and the southwestern Cape. (d) The Coastal biome is essentially that small part of the basin where climate is modified by proximity to the coast – the delta area and immediate hinterland. The dry season is not so marked, temperatures do not fluctuate greatly, and habitats include dry forest and grasslands. Most species found here are widespread along the East African coast from Somalia to northern KwaZulu-Natal. Table 1. Biomes of the Zambezi Basin. Zone percent Characteristics of basin Congolian 1.5 tropical; high rainfall; dry season not marked; mostly forest; links to Congo Basin Zambezian 95 sub-tropical; marked dry season; mostly moist or dry woodland & grassland Montane 2.5 temperate; some dry season precipitation; mostly moist forest, heath and grassland; part of Eastern Arc mountains Coastal 1 tropical; mild stable climate; mostly dry forest, woodland and grassland; part of East African coast 2.2 Evolution of the Basin Many of the patterns seen today in the distribution of species within the Zambezi Basin are the result of historical processes that have occurred over the last few million years (see Timberlake 1998). So an understanding of present-day biodiversity needs to be based on knowledge of changes in the climate, hydrology and geomorphology which have occurred within the region Biodiversity of the Zambezi Basin 4 over this period, as well as the modifications to the environment caused by human activity over the last thousand years or so. Changes in continental climate during the Pleistocene period (2 million years ago to present), caused in part by the Ice Ages in the Northern Hemisphere, have resulted in long periods of aridity across the basin interspersed with cooler periods with higher rainfall. Forest and forest species, which require cooler and moister conditions than are generally found in the basin today, would have been more widespread and not isolated patches or populations as at present, while the extent of savanna woodland would probably have been significantly less. And some areas would have changed less than others due to ameliorating factors such as altitude or proximity to the ocean. Thus the patterns of habitats and species we see today reflect, in part, the greater extent of forest during cooler, wetter periods, the more tropical and stable climate associated with the coastal plain, and the results of the successive waves of dry woodland that washed over the plateau. In addition to these historical factors there is, of course, a great variety of soils, moisture conditions, temperatures and landscape, all of which are reflected in different habitats, vegetation types and species composition. Owing to a series of river captures over the last few million years, the Zambezi landscape has been much modified, and this hydrological evolution of the river system is often reflected in species distribution (Timberlake 1998), particularly that of aquatic species or those dependent on water bodies. The present-day Zambezi system can be divided up into three separate sections, each differing in its landscape characteristics, its geological history and its biodiversity. These are the Upper Zambezi, the Middle Zambezi, and the Lower Zambezi. The Lake Malawi catchment is often considered part of the Middle Zambezi, although it does have its own unique characteristics. The Upper Zambezi is geomorphologically the oldest part of the basin and has probably not changed substantially for some 2-5 million years. The early Zambezi River, the proto-Zambezi, drained a vast area of the Central African Plateau, including the Bangweulu Swamps and Lake Mweru in northern Zambia. The Kafue, Kavango and other rivers drained towards the present- day Caprivi, where a series of huge inland lakes were thought to exist (Shaw & Thomas 1988, Thomas & Shaw 1988, 1991). Waters overflowing from here flowed into what are now the Shashe and Limpopo rivers, and exited to the sea north of present-day Maputo. Between 1.5 and 0.5 million years ago, a series of river captures took place in northeastern Botswana owing to slight earth movements, which eventually resulted in all the waters from the Upper Zambezi flowing eastwards into the Gwembe Trough – occupied today by Lake Kariba. Perhaps somewhat earlier than this the Chambeshi River (which now goes on to form the Bangweulu Swamps) and other headwaters were "captured" by aggressive southern tributaries of the Congo. Biodiversity of the Zambezi Basin 5 This vast inland drainage basin was, and is, geomorphologically mature with little relief. Species have few biological barriers to cross, and wetland organisms in particular could move along slow waterways and broad floodplains over a large part of the sub-continent. The Middle Zambezi, that part of the basin between Victoria Falls and the Lupata Gorge downstream of Tete in Mozambique, is much younger, more environmentally heterogeneous, and much smaller in extent than the Upper Zambezi. It includes the Luangwa Valley and most of northern Zimbabwe, although not the Kafue Flats. Lake Malawi, its catchment and the Upper and Middle Shire, are often considered to be part of the Middle Zambezi, although they are a biologically separate unit as regards aquatic biodiversity. Prior to the capture of the Upper Zambezi, this section would have been relatively small, typified by narrow floodplains, rapids and large fluctuations in seasonal flow, with a wide range of vegetation types from evergreen forests on the flanks of the high peaks through mesic woodlands on the watershed and higher ground, to drought-adapted deciduous woodlands (mopane and acacia) in the lower-lying broad valleys. Today, it is a dissected landscape, ranging from mountains and other high land down to broad dry valleys typically covered in mopane and other forms of drought-adapted deciduous woodland. Its biological character has been greatly modified in recent years by the construction of two huge dams – Kariba and Cabora Bassa – and a series of smaller dams on the tributaries, particularly in Zimbabwe. The Lower Zambezi extends from the Lupata Gorge in Mozambique to the coast where the river exits at Chinde, and also includes those parts of the Shire River in Malawi below the Kapichira Falls near Blantyre. This section of the Zambezi is typified by a broad floodplain, often with many parallel channels and shifting sandbanks, while the coastal portion includes extensive grasslands and freshwater swamps, dunes and mangroves. Although its geological history is not clear, this part of the basin has biological similarities to the Pungwe/Buzi system, and it is linked during floods to the Gorongosa area and Urema Trough. 2.3 Biodiversity Studies The biodiversity of the basin as a whole has not been comprehensively documented. Studies have been concerned principally with individual groups or with individual countries rather than the whole basin. However, a comprehensive review of various animal and plant groups over southern Africa, including the Zambezi Basin, was compiled by Werger (1978), and this remains perhaps the most valuable document to date.